There has been increasing demands for higher capacity secondary batteries in response to miniaturization of electronic devices driven by rapidly advancing development in the industry. This has prompted development of the lithium secondary batteries having higher energy density than nickel-cadmium batteries and nickel-hydrogen batteries. There has also been repeated effort for improving performance.
Against the background of increasing global challenges such as environmental and energy problems, there are high expectations for the application of lithium secondary batteries to large power supplies such as car power supplies and stationary power supplies. However, because such batteries are generally intended for use in environments exposed to ambient air, battery characteristics, particularly low-temperature discharge characteristics under low-temperature environment such as below freezing point are considered important in battery development. Further, because of its use, such batteries are required to have better life performance than conventional lithium secondary batteries.
The main components of the lithium secondary batteries are the positive electrode, the negative electrode, the separator, and the electrolytic solution. The electrolytic solution is typically a nonaqueous electrolytic solution produced by dissolving an electrolyte such as LiPF6, LiBF4, LiClO4, LiCF3SO3, LiAsF6, LiN(CF3SO2)2, and LiCF3(CF2)3SO3 in a nonaqueous solvent such as cyclic carbonate (ethylene carbonate, propylene carbonate, and the like), chain carbonate (dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, and the like), cyclic ester (γ-butyrolactone, γ-valerolactone, and the like), and chain ester (methyl acetate, methyl propionate, and the like).
There are various studies of nonaqueous solvent and electrolyte to improve the low-temperature discharge characteristics and the cycle characteristics of the lithium secondary batteries. For example, Patent Document 1 describes using a vinyl ethylene carbonate compound-containing electrolytic solution to minimize degradation of the electrolytic solution and produce a battery of excellent storage characteristics and cycle characteristics. Patent Document 2 describes using a propane sultone-containing electrolytic solution to increase the recover capacity after storage.
Patent Document 3 discloses using an electrolytic solution that contains a cyclic sulfonic acid ester having an unsaturated bond to fabricate a battery that can suppress degradation of the electrolytic solution even under high-temperature environment.
However, while containing these compounds provides some effect of improving storage characteristics and cycle characteristics, they form a high-resistance coating on the negative electrode side and lower the low-temperature discharge characteristics.
In an effort to improve the low-temperature discharge characteristics of the lithium secondary batteries, there have been efforts to suppress the reaction resistance of the system in a low-temperature discharge state by addition of a specific compound.
In Patent Document 4, there is a report of adding a silicone-based defoaming agent to the electrolytic solution to improve the low-temperature discharge capacity.
In Patent Documents 5 to 7, there are reports of suppressing the low-temperature internal resistance by using a technique whereby a silicon compound having an unsaturated bond is added to the electrolytic solution.
In Patent Document 8, there is a report of using a negative electrode containing Si, Sn, and the like as a main component, and adding an ethylene carbonate derivative and a predetermined Si-containing compound to the electrolytic solution to suppress battery swelling and improve cycle life.
Patent Documents 9 and 10 introduce techniques whereby hexamethyldisilane is added as an additive to reduce the irreversible capacity at the negative electrode, and suppress the degradation reaction of the electrolytic solution at the negative electrode.
In Patent Documents 11 and 12, there are reports of adding a phosphazene derivative to the electrolytic solution to suppress the interface resistance of the electrolytic solution and improve low-temperature discharge characteristics.
In Patent Documents 13 to 15, there are reports of improving low-temperature discharge characteristics by using a technique whereby a predetermined phosphoric acid compound is added to the electrolytic solution.